It's valid, and on a DONE4 you fly in A on 2-class flights within the USA if available.

You were clearly taling to someone at BA who has no clue. For ex-SA xONEn tickets, set them up through the AA RTW desk and then ticket either through Mindpearl in SA or an AA office in the EU or Canada, whichever is more convenient for you. There's no reason to talk to BA.